It has long been known that the unique crystalline structure of graphite makes it anisotropic with respect to electrical charge carriers. Its structure basically comprises planes of hexagonally arrayed aromatically bound carbon atoms. Hence, each of such planes has .pi. clouds of electrons above and below it. These electron clouds have been said to contribute to its anisotropic conductive behavior, the higher conductivity being in the direction parallel to the aromatic carbon plates. This conductivity is approximately 5 percent that of copper, at best.
It also long has been known that it is possible to form carbonaceous fibrous materials which incorporate to at least some degree graphitic carbon. Such carbonaceous fibrous materials prior to intercalation can be formed by the thermal treatment of a variety of polymeric fibrous materials in accordance with procedures known in the art. See, for instance, the following commonly assigned U.S. Patents which disclose the formation of carbon fibers which include the presence of graphitic carbon beginning with an acrylic fibrous precursor (as defined): Nos. 3,656,904; 3,775,520; 3,818,082; 3,900,556; 3,925,524; and 3,954,950. Most of the commercially available carbon fibers available today are formed at a maximum temperature well below 2000.degree. C. It has been the practice heretofore rarely to form carbonaceous fibrous materials at maximum processing temperatures higher than approximately 2700.degree. to 2900.degree. C. since the production of any higher temperatures have been more difficult to achieve and control and more expensive to sustain over an extended period of time. The graphitic carbon present in such carbonaceous fibrous materials has been turbostratic (i.e., the graphitic basal planes have tended to be parallel but randomly oriented with respect to the crystallographic a.sub.1 and a.sub.2 axes of the hexagonal lattice). When subjected to wide angle x-ray analysis such heretofore produced fibers derived from an acrylic fibrous precursor have exhibited a single diffraction peak comprising unresolved Miller index (100, 101) reflections and the absence of a (112) reflection. U.S. Pat. No. 4,005,183 discloses carbon fibers containing graphitic carbon which are derived from pitch which when subjected to wide angle x-ray analysis exhibit resolved Miller index (100) and (101) reflections and the presence of a (112) reflection. It is stated in this patent that at Col. 3, lines 38 et seq., that fibers derived from the processing of acrylic fibers to 2500.degree. to 3000.degree. C. and higher exhibit unresolved Miller index (100, 101) reflections and the absence of a (112) reflection.
It further has been recognized that certain elements or molecules, when diffused into a graphite lattice, assume positions interstitial to the aromatic planes and improve graphite conductivity. Such positioning of elements or molecules within the graphitic carbon structure has been termed "intercalation" and commonly has produced a reduced electrical resistivity. Ubbeholde, for example, found that the interstitial compound formed between individual graphite crystals and nitric acid has a volume conductivity almost equal to that of copper (which is approximately 0.6.times.10.sup.6 ohms.sup.-1 cm..sup.-1) when measured parallel to the aromatic planes (A. R. Ubbeholde, Proc. Roy. Soc., A304, 25, 1968). The following are additional representative publications which concern the intercalation of graphite: U.S. Pat. Nos. 3,962,133; 3,984,352; 3,409,563; 4,035,434; 4,083,885; and 4,119,655; "Rare Earth Graphite Intercalation Compounds" by W. E. Craven and W. Ostertag appearing in Carbon, Vol. 4, pages 223-226 (1966); "Graphite Intercalation Compounds With Chlorides of Manganese, Nickel and Zinc" by E. Stumpp and F. Werner appearing in Carbon, Vol. 4, page 538 (1966); "High Electrical Conductivity in Graphite Intercalated With Acid Fluorides" by F. L. Vogel, G. M. T. Foley, C. Zeller, E. R. Falardeau and J. Gan appearing in Materials, Science and Engineering, Vol. 31, pages 261-265 (1977), "The Electrical Conductivity of Graphite Intercalated With Superacid Fluorides: Experiments With Antimony Pentafluoride" by F. L. Vogel appearing in Journal of Materials Science, Vol. 12, pages 982-986 (1977); "Very High Electrical Conductivity in AsF.sub.5 -Graphite Intercalation Compounds" by E. R. Falardeau, G. M. T. Foley, C. Zeller, and F. L. Vogel appearing in Journal of the Chemical Society, Chemical Communications, pages 389-390 (1977); "Chemistry of Graphite Intercalation by Nitric Acid" by W. C. Forsman, F. L. Vogel, D. E. Carl and J. Hoffman appearing in Carbon, Vol. 16, pages 269-271 (1978); and "Charge Transfer in Graphite, Nitrate and the Ionic Salt Model" by S. Loughin, R. Grayeski, and J. E. Fisher appearing in J. Chem. Phys. 69(8), pages 3740-3743 (1978).
Additionally, it is known that carbonaceous fibrous materials containing graphitic carbon can be intercalated to form a fibrous product of reduced electrical resistivity. However, it heretofore has not been possible to reduce the electrical resistivity of such carbon fibers via intercalation to the low levels achievable with other forms of graphite such as individual graphite single crystals or highly oriented pyrolytic graphite (HOPG). Such inability to achieve extremely high levels of electrical conductivity is believed to be traceable to at least some degree to the turbostratic nature of the graphitic carbon crystallites inherently present in such fibers (i.e., the lack of orientation within the parallel layers of the crystallites comprising the fiber).
The following are representative disclosures which discuss at least in part the formation of a graphite intercalation compound within a carbonaceous fibrous material: "Interstitial Compounds of Potassium With Carbon Fibers" by C. Herinckx, R. Perret and W. Ruland appearing in Carbon, Vol. 10, pages 711-722 (1972); German Pat No. 2,537,272; "Electrical Resistivity of Nitrate Intercalated Graphite Fibers" by F. L. Vogel appearing in the Proceedings, 4th London International Conference on Carbon and Graphite (1976); U.S. Ser. No. 897,443, filed Apr. 18, 1978; and "The Intercalation of Bromine in Graphitized Carbon Fibers and Its Removal" by J. G. Hooley and V. R. Deitz appearing in Carbon, Vol. 16, pages 251-257 (1978).
It is an object of the present invention to provide an improved process for the intercalation of graphitic carbon fibers.
It is an object of the present invention to provide an improved carbonaceous fibrous material containing graphitic carbon which is particularly suited for intercalation.
It is an object of the present invention to provide improved intercalated graphitic carbon fibers having an extremely low specific electrical resistivity which in a preferred embodiment is no greater than that of copper.
It is an object of the present invention to provide improved intercalation graphitic carbon fibers which advantageously exhibit desirable tensile properties such as strength and initial modulus in addition to a reduced electrical resistivity.
It is an object of the present invention to provide improved intercalated graphitic carbon fibers which because of their extremely low specific electrical resistivity and highly satisfactory tensile properties can be utilized to particular advantage as lightweight electrical conductors.
These and other objects, as well as the scope, nature, and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.